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Newbie here. I've found the following rules to set a firewall, (from the book how Linux works)

iptables -P INPUT DROP   # the default policy
iptables -A INPUT -s 127.0.0.1 -j ACCEPT     
iptables -A INPUT -p tcp '!' --syn -j ACCEPT   # accepting incoming 
connections from everywhere except those initiating a connection hence syn

So far so good (or it seems to be). The trouble comes when I try to add a rule for DNS, here's what I have tried and didn't seem to get it right(one at a time):

INPUT -p udp --dport 53 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p udp --source-port 53 -s 127.0.1.1 -j ACCEPT
iptables -A INPUT -p udp --source-port 53 -j ACCEPT

Desired result: Preventing any initialization of connections from outside (ssh, icmtp, ... ), enabling DNS lookup and web browsing (curl, wget, telnet ...), I don't think it is relevant that I may locally run a web server or a database server ...

Any help would be appreciated.

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I suggest you create some Input rule that allows all established and related traffic like:

$IPTABLES -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Also you should always allow your loopback-device:

# Allow loopback interface to do anything.
$IPTABLES -A INPUT -i lo -j ACCEPT
$IPTABLES -A OUTPUT -o lo -j ACCEPT

If your default output policy is accept, most problems should be gone. Otherwise you should also add:

$IPTABLES -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

and open the ports for HTTP(s), DNS, ICMP or whatever you need.

https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers

https://en.wikibooks.org/wiki/Communication_Networks/IP_Tables

  • Can you tell me please why is it important to make the loopback related instructions, and if the instruction in the question with --syn option doesn't already allow established connections?? – user10089632 Feb 22 '18 at 12:39
  • and equally important I need the instruction that allow DNS please, as I've mentioned earlier. – user10089632 Feb 22 '18 at 12:41
  • The INPUT ESTABLISHED,RELATED rule should take care of your DNS issues. – Doug Smythies Feb 22 '18 at 15:25
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    @user10089632 I am no expert for iptables. I just wanted to give you a working solution. The Loopback device 127.0.0.1 can be used by software on your computer to communicate with other software internally (installed on your computer). check sudo netstat -tulpn to print information about the Linux networking subsystem. Also it's a good idea to tell iptables to log droped, or rejected packets, so you know about what is beeing blocked. – AlexOnLinux Feb 26 '18 at 7:46
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Allowing Established Sessions We can allow established sessions to receive traffic:

sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

If the line above doesn't work, you may be on a castrated VPS whose provider has not made available the extension, in which case an inferior version can be used as last resort:

sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Allowing Incoming Traffic on Specific Ports

sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT

Blocking Traffic

sudo iptables -A INPUT -j DROP

Enabling loopback by editing iptables :

sudo iptables -I INPUT 1 -i lo -j ACCEPT

Logging of unwanted traffic:

sudo iptables -I INPUT 4 -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

To confirm that changes have been successfully made:

iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh
LOG        all  --  anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
DROP       all  --  anywhere             anywhere

Use iptables -L -v to get more details :

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     all  --  lo     any     anywhere             anywhere
    0     0 ACCEPT     all  --  any    any     anywhere             anywhere            state RELATED,ESTABLISHED
    0     0 ACCEPT     tcp  --  any    any     anywhere             anywhere            tcp dpt:ssh
    0     0 LOG        all  --  any    any     anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables denied: "
    0     0 DROP       all  --  any    any     anywhere             anywhere

Empty iptables:

iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -X

Saving iptables

If you were to reboot your machine right now, your iptables configuration would disappear. Rather than type this each time you reboot, however, you can save the configuration, and have it start up automatically.

Save your firewall rules to a file

sudo sh -c "iptables-save > /etc/iptables.rules"

The script /etc/network/if-pre-up.d/iptablesload will contain:

#!/bin/sh
iptables-restore < /etc/iptables.rules
exit 0

and /etc/network/if-post-down.d/iptablessave will contain:

#!/bin/sh
iptables-save -c > /etc/iptables.rules
if [ -f /etc/iptables.downrules ]; then
   iptables-restore < /etc/iptables.downrules
fi
exit 0

Then be sure to give both scripts execute permissions:

sudo chmod +x /etc/network/if-post-down.d/iptablessave
sudo chmod +x /etc/network/if-pre-up.d/iptablesload

Source

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